1. Field of the Invention
The present invention relates to fluxgates and, more particularly, to a semiconductor fluxgate magnetometer.
2. Description of the Related Art
A magnetometer is a device that measures the strength of an external magnetic field. There are a number of different approaches to measuring magnetic fields, and various different types of magnetometers have been developed based on these different approaches. One type of magnetometer based on flux variations in a magnetic core is a fluxgate magnetometer.
FIG. 1 shows a block diagram that illustrates an example of a prior art fluxgate magnetometer 100. As shown in FIG. 1, fluxgate magnetometer 100 includes a drive coil 110, a sense coil 112, and a magnetic core structure 114 that lies within drive coil 110 and sense coil 112.
As further shown in FIG. 1, fluxgate magnetometer 100 also includes a drive circuit 120 that is connected to drive coil 110, and an sense circuit 122 that is connected to sense coil 112 and drive circuit 120. Sense circuit 122 generates an output voltage VOUT that is proportional to the magnitude of an external magnetic field.
FIGS. 2A-2E show views that illustrate the operation of fluxgate magnetometer 100. FIG. 2A shows a graph that illustrates a BH curve 200 for magnetic core structure 114, while FIG. 2B shows a graph that illustrates an alternating current input to drive coil 110, FIG. 2C shows a graph that illustrates the magnetic induction field B that results from the alternating current input to drive coil 110, FIG. 2D shows a graph that illustrates an induced voltage in sense coil 112 plotted in the time domain that results from the magnetic induction field B, and FIG. 2E shows a graph that illustrates the induced voltage in sense coil 112 plotted in the frequency domain that results from the magnetic induction field B.
As shown by BH curve 200 in FIG. 2A, when the magnitude of a magnetic field H increases, magnetic core structure 114 increases the magnitude of the magnetic induction field B until magnetic core structure 114 saturates. Once in saturation, further increases in the magnitude of the magnetic field H lead to very little increase in the magnitude of the magnetic induction field B.
As a result, saturation is commonly illustrated as in FIG. 2A as the region where increases in the magnitude of the magnetic field H lead to no additional increase in the magnitude of the magnetic induction field B. In the present example, the magnitude of the magnetic field H is increased by increasing the magnitude of the alternating current flowing through drive coil 110.
As shown in FIGS. 2A-2C, when no external magnetic field is present and an alternating current waveform 210, which has an amplitude that is sufficient to drive magnetic core structure 114 into saturation, is input to drive coil 110 from drive circuit 120, an alternating magnetic induction field B, as represented by waveform 212, is generated in response.
In other words, when alternating current waveform 210 is input to drive coil 110, magnetic core structure 114 is driven through cycles (magnetized, un-magnetized, inversely magnetized, un-magnetized, magnetized again, and so on) that generate an alternating magnetic induction field B as represented by waveform 212. In the present example, the alternating current waveform 210 is triangular, while the magnetic induction field waveform 212 has flat tops and bottoms that represent the periods of saturation.
As shown in FIG. 2D, the alternating magnetic induction field B induces an alternating voltage 214 in sense coil 112. The induced alternating voltage 214 is proportional to the change in the magnetic induction field B over time (dB/dt). In addition, the induced alternating voltage 214 is processed by sense circuit 122 to generate the output voltage VOUT, which is proportional to the external magnetic field.
As shown in FIG. 2E, in the frequency domain, the induced alternating voltage 214 has a fundamental frequency 1f, but only odd harmonics, such as a third harmonic 3f, of the fundamental frequency 1f. As a result, when no external magnetic field is present, the induced alternating voltage 214 has no second harmonic.
However, when an external magnetic field is present, the external magnetic field interacts with magnetic core structure 114 and changes the alternating magnetic induction field B. In other words, magnetic core structure 114 is more easily saturated when magnetic core structure 114 is in alignment with the external magnetic field, and less easily saturated when magnetic core structure 114 is in opposition to the external magnetic field.
In the present example, as shown by waveform 220 in FIG. 2C, alignment to the external magnetic field increases the duration of the positive magnetic induction field B and decreases the duration of the negative magnetic induction field B. As a result, as shown by waveform 222 in FIG. 2B, the external magnetic field has the effect of shifting alternating current waveform 210 to the right.
In other words, when no external magnetic field is present, each half cycle of the waveform 210 drives magnetic core structure 114 into positive and negative saturation by substantially an equal amount. However, when exposed to an external magnetic field, as illustrated by the waveform 222, the external magnetic field causes one half-cycle of the waveform 222 to drive magnetic core structure 114 more deeply into saturation, and one half-cycle of the waveform 222 to drive magnetic core structure 114 less deeply into saturation.
In addition, as shown in FIG. 2D, the change in the alternating magnetic induction field B phase shifts the induced alternating voltage 214 to generate a phase-shifted induced alternating voltage 224. Further, as shown in FIG. 2E, in the frequency domain, the phase-shifted induced alternating voltage 224 that results from the external magnetic field includes even harmonics, specifically a second harmonic 2f. 
The magnitude of the second harmonic 2f, in turn, is proportional to the magnitude of the external magnetic field. Thus, by filtering the phase-shifted induced alternating voltage 224 to isolate the second harmonic 2f, and then detecting the magnitude of the second harmonic 2f, the magnitude of the external magnetic field can be determined.
FIG. 3 shows a block diagram that illustrates an example of a prior art fluxgate magnetometer 300. Fluxgate magnetometer 300 is similar to fluxgate magnetometer 100 and, as a result, utilizes the same reference numerals to designate the structures which are common to both fluxgate magnetometers.
As shown in FIG. 3, fluxgate magnetometer 300 differs from fluxgate magnetometer 100 in that fluxgate magnetometer 300 includes a feedback coil 310 that is wrapped around magnetic core structure 114 along with drive coil 110 and sense coil 112, and a feedback circuit 312 that is connected to feedback coil 310 and sense circuit 122. Feedback circuit 312 generates an output voltage VCAN that has an amplitude which is proportional to the magnitude of an external magnetic field.
Fluxgate magnetometer 300 operates the same as fluxgate magnetometer 100, except that feedback coil 310 and feedback circuit 312 are utilized to generate a magnetic field that opposes the external magnetic field. FIG. 2C shows that alignment to the external magnetic field increases the duration of the positive magnetic induction field B and decreases the duration of the negative magnetic induction field B.
In addition, as the strength of the external magnetic field increases, the duration of the positive magnetic induction field B increases while the duration of the negative magnetic induction field B decreases. Thus, as the strength of an external magnetic field increases, the duration of the negative induction field B decreases until the fluxgate magnetometer reaches saturation where there is substantially no negative magnetic induction field B. Once the fluxgate magnetometer saturates, further increases in the strength of the external magnetic field can not be detected by the fluxgate magnetometer.
To prevent a strong external magnetic field from saturating a fluxgate magnetometer, the alternating current input to feedback coil 310 is selected to generate a magnetic field that cancels out the external magnetic field, and effectively make the output voltage VOUT appear as though no external magnetic field were present.
The magnitude of the alternating current input to feedback coil 310 when the output voltage VOUT appears as though no external magnetic field were present can then be used to generate the output voltage VCAN. Since the amplitude of the output voltage VCAN is proportional to the magnitude of the external magnetic field, the magnitude of the external magnetic field can then be determined. Thus, the advantage of fluxgate magnetometer 300 is that fluxgate magnetometer 300 can be used in very strong magnetic fields.
FIG. 4 shows a block diagram that illustrates an example of a prior art fluxgate magnetometer 400. As shown in FIG. 4, fluxgate magnetometer 400 includes a drive coil 410, a sense coil 412, and a pair of magnetic core structures 414 and 416 that lie within drive coil 410 and sense coil 412.
As further shown in FIG. 4, fluxgate magnetometer 400 also includes a drive circuit 420 that is connected to drive coil 410, and a sense circuit 422 that is connected to sense coil 412. Sense circuit 422 generates an output voltage VDIF that has an amplitude which is proportional to the magnitude of an external magnetic field.
In operation, drive coil 410 is wrapped around the magnetic core structures 414 and 416 so as to generate equal and opposing alternating magnetic induction fields when drive circuit 420 outputs an alternating current to drive coil 410. Thus, when no external magnetic field is present, no voltage is induced in sense coil 412 because no alternating magnetic induction field is present.
FIG. 5 shows a graph that further illustrates the operation of fluxgate magnetometer 400. As shown in FIG. 5, although no current is induced in sense coil 412 when no external magnetic field is present, the presence of an external magnetic field induces an alternating voltage in sense coil 412.
Sense circuit 422 detects the induced alternating voltage in sense coil 412 and generates in response the output voltage VDIF, which has an amplitude that is proportional to the magnitude of the external magnetic field. Thus, by detecting the amplitude of the output voltage VDIF, the magnitude of the external magnetic field can be determined.
One of the advantages of fluxgate magnetometer 400 over fluxgate magnetometer 100 is that fluxgate magnetometer 400 requires less support circuitry than fluxgate magnetometer 100. For example, drive circuit 120 commonly generates a second harmonic clock signal which drive circuit 410 need not generate. The second harmonic clock signal, which has a frequency that is equal to the second harmonic of the fundamental frequency of the clock signal used to input current to drive coil, is typically required by sense circuit 122.
FIG. 6 shows a block diagram that illustrates an example of a prior art fluxgate magnetometer 600. Fluxgate magnetometer 600 is similar to fluxgate magnetometer 100 and, as a result, utilizes the same reference numerals to designate the structures which are common to both fluxgate magnetometers.
As shown in FIG. 6, fluxgate magnetometer 600 differs from fluxgate magnetometer 100 in that fluxgate magnetometer 600 utilizes a magnetic core structure 610 in lieu of magnetic core structure 114. Magnetic core structure 610 differs from magnetic core structure 114 in that magnetic core structure 610 has flared ends. Fluxgate magnetometer 600 operates the same as fluxgate magnetometer 100 except that the flared ends of magnetic core structure 610 capture additional flux and channel the additional flux into the body of magnetic core structure 610, thereby functioning as a flux concentrator.
FIG. 7 shows a block diagram that illustrates an example of a prior art fluxgate magnetometer 700. Fluxgate magnetometer 700 is similar to fluxgate magnetometer 100 and, as a result, utilizes the same reference numerals to designate the structures which are common to both fluxgate magnetometers.
As shown in FIG. 7, fluxgate magnetometer 700 differs from fluxgate magnetometer 100 in that fluxgate magnetometer 700 utilizes a magnetic core structure 710 in lieu of magnetic core structure 114. Magnetic core structure 710 differs from magnetic core structure 114 in that magnetic core structure 710 has a narrow center section. In addition, fluxgate magnetometer 700 differs from fluxgate magnetometer 100 in that sense coil 112 of fluxgate magnetometer 700 is only wrapped around the narrow center section of magnetic core structure 710.
Fluxgate magnetometer 700 operates the same as fluxgate magnetometer 100 except that the narrow section of magnetic core structure 710 saturates faster than the remaining sections of magnetic core structure 710. As a result, less current is required to saturate the section of magnetic core structure 710 that is wrapped by sense coil 112.
Although the fluxgate magnetometers 100, 300, 400, 600, and 700 each measures the strength of an external magnetic field, the fluxgate magnetometers 100, 300, 400, 600, and 700 tend to be bulky and expensive to manufacture. Thus, there is a need for a smaller and less expensive fluxgate magnetometer.